Okay, so this is basically telling you that there is no solution. You don't have to worry about there being one. You just have to justify (or explain) why there isn't one.
Basically, neither y or x is equal to anything. You couldn't make x equal to a value in y because x and y are both together in the equation and equal to numbers.
We can always move a factor to the other side. So,
x-2y= -8
Add 2y to the other side.
x= -8+2y
Now, plug in -8+2y as x.
3(-8+2y)- 6y=-12
-24+6y-6y= -12
The 6y's cancel out. So, we can't figure out what y is.
-24=-12 <- no y.
These two lines are parallel. They have direct proportion in each other. That is why there is no solution because the lines never cross. Parallel lines do not cross.
I hope this helps!
~kaikers